There are instances when it is necessary to determine moisture levels on a surface such as the windscreen or windshield of a vehicle.
In the case of the windscreens of road vehicles, there has been a number of attempts in the past to determine the need to wipe the windscreen by determining the current level of rainfall or the current level of water on the windscreen. Some of these attempts, such as is described in U.S. Pat. No. 4,355,271 (Noack), have used devices which emit radiant energy from the inside of the windscreen, and direct it at the outside windscreen surface at an angle such that it will be substantially totally internally reflected from a dry screen but allowed to substantially entirely pass through a wet screen. By suitably positioning a detector that will respond to the radiant energy, it can be determined whether or not the screen is wet. However, typical examples of such devices are inefficient due to reflective losses incurred when the radiant energy beam enters and leaves the windscreen at the inside air/screen interface at a similar angle. U.S. Pat. No. 4,620,141 (McCumber et al) discloses a sensor unit of this general type but further including a light conducting rod for each emitter and detector and may overcome the problem of reflective losses. Such devices tend to be less than straight forward to manufacture and assemble. Furthermore, the required physical dimensions of the prior art devices are inconveniently large in order to obtain the required relative location of the sensor and emitter.